When nonfree/nontrivial code is detected in a page, LibreJS attempts to find a relevant contact link or email for the website you are visiting. In order to do so, it will attempt to visit a few links from the current page (for instance, a link labeled “contact” on the same domain as the current page, ...)
LibreJS can currently detect and list contact pages with webforms, email addresses more likely to be owned by the maintainer of the site, Twitter and identi.ca links, phone numbers.
After LibreJS detects one or more of the above, a small tab will appear on the right of your screen with the LibreJS torch logo. When hovering over the tab, a large panel will slide in with the contact information. Ideally, at the top you will find the email address of the maintainer, labeled as the “Email you should use”.